The present invention relates in general to diagnostic methods for electrical systems which are associated with motor vehicles. More specifically the present invention relates to the use of a switched load within an electrical assembly in order to distinguish between low voltage due to an inadequate power source as compared to low voltage due to a wiring harness problem.
High-current, electronically-controlled assemblies in vehicles require adequate voltage to function properly. An example is a "smart" fuel injection pump which has an on-board electronic control module that controls various solenoids based on various input signals. When the fuel control solenoid is energized, the current draw can range from about 10 amps to 20 amps. If too small a wire is used in the electrical harness, or corrosion increases contact resistance in the harness, the voltage at the fuel injection pump can drop unacceptably low. This can cause fueling problems, timing problems, low power, stalling, and no start. Given today's service environment, this situation can lead to fuel pumps being replaced, when the problem is really with the harness. This results in higher warranty costs and longer service times.
According to the present invention, a diagnostic method is described which can detect unacceptably high harness resistance before symptoms occur.